1. Field of the Invention
The invention relates to a halogen-free, flame-retardant insulating epoxy resin composition. In particular, the invention relates to a halogen-free, flame-retardant insulating epoxy resin composition used during the formation of wiring layers on a substrate formed mainly of an organic material, in the field of multi-layer circuit boards, such as MCM-L/D (multi-chip module-laminate/deposit) circuit boards or single-chip package circuit boards, and a multi-layer circuit board comprising insulation layers formed by the use of the epoxy resin composition.
2. Description of the Related Art
A printed circuit board has been commonly used to compactly incorporate electronic parts in electronic equipment. Such a printed circuit board, which is produced by etching a copper foil provided on a surface of a laminated sheet (a core substrate) following given electronic circuit patterns, and is used by mounting thereon various electric or electronic parts, is difficult to densely load with electronic parts, but has the advantage of low cost. For a hybrid IC, on the other hand, a build-up multi-layer wiring structure has been used for a long time. In this case, a multi-layer circuit board is fabricated by successively printing alternate thick paste films of a conductor and an insulator on a ceramic substrate to form a laminate, and then firing it.
Recently, due to a demand for miniaturization, enhanced performance, and reduced cost of electronic equipment, the miniaturization of and the use of multi-layer structure in electronic circuits, and the dense mounting of electronic parts have proceeded rapidly, and the use of build-up multi-layer wiring structure in printed circuit boards has been considered in the area of printed circuit boards. An insulation layer for a build-up structure is formed over an underlying wiring layer except for locations (portions of piercing hole, generally called “via hole”) at which the electronic circuits on and under the insulation layer are connected with each other.
To form the insulation layer for a build-up structure, an epoxy resin is mainly used because of its excellent electrical properties, workability, and cost.
In general, an epoxy resin has the shortcoming of being relatively flammable. This is not desirable for a material used in electronic equipment which must be difficult to burn. Accordingly, to make an epoxy resin flame retardant, it is most common to use a halogenated epoxy (particularly an epoxy containing bromine) and antimony oxide as a flame retardant in combination.
However, since a halogenated organic material, such as a halogenated epoxy, will generate a harmful substance having high toxicity during combustion, and antimony oxide is a carcinogenic substance, the epoxy resin composition using these in combination poses environmental and safety problems. Accordingly, the trend to discontinue use of products using such a flame-retardant epoxy resin composition has intensified, mainly in Europe. As a result, a halogen-free and antimony-free insulating resin which does not use both halogen-based and antimony-based flame retardants is required, and development of such a material is strongly demanded.
It is known to use a metal hydrate, particularly aluminum hydroxide or magnesium hydroxide, in an epoxy resin composition, in lieu of a halogen-based flame retardant. To develop practical flame retardant properties, however, it is required to add a metal hydroxide to an epoxy resin composition in a large amount, with the result that the composition has poor curing, insulating, and mechanical properties, and problems occur in the formation of build-up layers.
It is also known, in the field of flame retardants, to use phosphorus or an organic phosphorus compound as a phosphorus-based flame retardant. Phosphorus-based flame retardants are classified into an addition type and a reaction type. Representative of the phosphorus flame retardant of the addition type is phosphorus (red phosphorus) or a phosphate. However, in the case of the addition of phosphorus, there is a difficulty of degraded insulating properties of the epoxy resin composition as in the case of the metal hydrate. In the case of the use of a common phosphate, a phenomena of the phosphorus compound appearing at the surface of an insulation layer at an elevated temperature, which is called “bleedout”, poses a problem, and a phosphate has a shortcoming of being affected by a plating process.